Fe(II)/Fe(III) Catalysed Peroxymonosulfate Modelling in the Presence of Protocatechuic Acid

Phenolic compounds are persistent pollutants in agro-industrial wastewaters and hinder biological treatment due to their antimicrobial activity. Activation of peroxymonosulfate (PMS) by iron species represents an efficient advanced oxidation route for their degradation. In this work, the oxidation of protocatechuic acid (pCtchA) by the Fe(II)/Fe(III)/PMS system was investigated through a mechanistic model constructed from a comprehensive set of elementary reactions describing radical and non-radical pathways. The coupled kinetic equations were numerically solved to predict the temporal evolution of PMS, iron species, and organic intermediates. Model simulations were validated with batch reactor data and accurately reproduced the effects of iron and PMS concentrations, temperature, and pH. The results confirm that Fe(III) reduction to Fe(II) is the rate-determining step, and that the ortho-dihydroxy functionality of pCtchA promotes this reaction via Fe(III)–ligand complexation, sustaining the catalytic cycle. Optimal degradation occurs near pH 3.5, where Fe(III)–pCtchA monocomplexes predominate, and the apparent activation energy for Fe(III) reduction is approximately 36 kJ mol⁻¹. This study provides a quantitative kinetic framework for the Fe(II)/Fe(III)/PMS system and demonstrates its potential as a sustainable catalytic process for the oxidation of dihydroxy-substituted phenolic pollutants in aqueous media.